When the US Supreme Court issues a decision does it make it law?

Ex post facto law

The initial question is very complex. Thus, any simple answer will be wrong to some degree. Therefore, a somewhat academic discussion is required to explain a complicated answer. One must start with the separation of powers structure of the Constitution and, therefore, our government: the Legislative branch that makes laws; the Judicial branch that interprets, and the Executive branch that enforces the law.

The Constitution is the "law" of the land; i.e., all laws made by Congress, and actions taken by the other branches must conform to constitutional principles; but it is not the only law. I assume the questioner is referring to interpretations of the U.S. Constitution by the Supreme Court. The interpretations made by the Court are law, they are what the Constitution means, at least until overruled. Thus, together (the Constitution and the interpretations) constitute constitutional law. The Court's decisions control the actions of the government and the citizen, thus, they constitute laws. To say that only the Constitution is "law" is to assign metaphysical content to the term. One cannot separate the Constitution as written from the interpretations made of it in Court decisions. Thus, the important idea is what is meant by the term "law." In an abstract or metaphysical sense, the Constitution is "the law", and the Court merely interprets or says what the Constitution has meant (or said) all along. In reality, the Court gives meaning to the Constitution and thus does "make law."

Note that one would say the same of statutes. However, the Congress can overrule the Court's interpretation of a statute.

Finally, there is a body of common law (federal and state). Common law, which is most of our laws, is judge-made law; e.g., tort law, contract law, criminal law, and evidence to name a few. Much of this has been codified and is now statutory, but much has not been codified. However, the Supreme does not get involved in this body of law unless it contravenes some Constitutional provision. An example is whether hearsay rules of evidence (common law) violate a defendant's Sixth Amendment rights (Constitutional law). The Court does not say what the common law should be, just whether or not it violates the Constitution. If so, the common law must be changed by the lower courts to reflect the Supreme Court's constitutional ruling. Rules of procedure are mostly made by courts, but also have in part been codified. But note that procedural law (rules) are often more important than substantive laws (statutes).

Precedents arise from the legal principles created by a court decision. In general, decisions of higher courts within a system are mandatory precedent on lower courts within that same system. The principle announced in the decision must be followed in later cases. Cases need not be identical or similar to control. The principle announced in a case transcends the particular facts in that case and will be applied to different fact situations. However, a decision may be grounded in the specific facts and will not be applied generally. Knowing the difference is what law school is all about. The process of judicial decision making, the use and role of precedents, constitutional and statutory interpretations, is what a legal education teaches.